1. FIELD OF THE INVENTION
The present invention relates to a vibration detecting device for detecting knocking by the detection of vibrations in, for instance, an internal combustion engine.
2. DISCUSSION OF BACKGROUND
FIGS. 5a and 5b are respectively a plan view and a cross-sectional view taken along a line II--II in FIG. 5a which show a conventional vibration detecting device. In FIGS. 5a and 5b, a reference numeral 1 designates a bush made of metal which has a bolt-insertion hole 13 at its center. The bush 1 also has a flange and a cylindrical portion 1c around which structural elements are assembled. The flange of the bush 1 has a seating surface 1a which is to be in contact with an internal combustion engine when the vibration detecting device is mounted on it and a base surface 1b on which the structural elements are stacked in the vertical direction. A numeral 2 designates a casing of a resinous material which is formed by insertion-molding or injection along with the bush 1 so as to form a receiving portion at the outer side of the cylindrical portion of the bush 1. The casing 2 also constituted a housing of the vibration detecting device. A numeral 3 designates a connector extending from a part of the casing 2 outwardly and having an output terminal 11 for outputting a detection signal. A numeral 4 designates a plate placed on the base surface 1b of the flange of the bush 1. A numeral 5 designates a piezoelectric element which converts vibrations into an electric signal. The piezoelectric element 5 has its reference electrode connected to the plate 4. A numeral 6 designates a terminal electrically connected to the output electrode of the piezoelectric element 5 to output a detection signal. A numeral 7 designates an insulating sheet made of a film of polyethylene terephthalate (PET) or polyphenylene sulfite (PPS) or the like which is disposed on the opposite side of the terminal 6 which is in contact with the piezoelectric element 5. A numeral 8 designates a weight which applies a vibrating stress to the piezoelectric element 5. A numeral 9 designates a nut fastened to a threaded portion formed in the outer circumference of the cylindrical portion 1c of the bush 1 in order to fit structural elements of the vibration detecting device such as the plate 4, the piezoelectric element 5, the terminal 6, the insulating sheet 7 and the weight 8. A numeral 12 designates an insulating tube made of an adhesive tape or a heat-shrinkable tube which is interposed between the bush 1 and the structural elements. The detection signal obtained at the piezoelectric element 5 is outputted at the output terminal 11 via the terminal 6 and the connector 3.
FIGS. 6a and 6b are respectively diagrams showing examples of the frequency characteristics obtained by the conventional vibration detecting device as shown in FIG. 5.
FIG. 7a is a longitudinal cross-sectional view of another conventional vibration detecting device. The same reference numerals as in FIG. 5 designate the same or corresponding parts. In the conventional vibration detecting device, a cushioning material 21 is placed between the outer circumference of the weight 8 and the inner circumference of the casing 2 to weaken the mechanical connection between them. As a material for the casing, polybutylene terephthalate (PBT) is generally used because it is relatively hard and it is excellent in transmitting vibrations in a vibration detecting system.
FIG. 7b is a plan view showing a state of the weight 8 assembled with the cushioning material 21. In this case, the cushoning material 21 does not extend to the contacting portion between the output terminal 11 and the terminal 6.
FIG. 8 is a diagram showing an example of the frequency characteristic of the conventional vibration detecting device as shown in FIG. 7.
In the following, the principle of detecting vibrations will be described. The vibration detecting device is fixed to an internal combustion engine by means of a bolt (not shown) inserted through the bolt-insertion hole 13 of the bush 1 so that the seating surface 1a is in contact with a portion of the internal combustion engine. Vibrations result depending on the operational condition of the internal combustion engine and are transmitted to the vibration detecting device via the seating surface 1a. Since the piezoelectric element 5 and the weight 8 are assembled in an overlapping state on the bush 1, an inertial stress in the weight 8 is transmitted to the piezoelectric element 5 depending on the vibrations. When the piezoelectric element 5 receives the inertial stress, a detection signal in proportion to the magnitude of the inertial stress is produced so that a detection signal is outputted to the output terminal of the electrode. The intensity of the detection signal is based on a potential at the plate side electrode of the piezoelectric element 5. Because the plate 4 is made of metal, the potential at the reference electrode of the piezoelectric element 5 is the same as the bush 1. On the other hand, the potential at the weight 8 is the same as the reference electrode of the piezoelectric element 5 because the nut 9 and the weight 8 are made of metal. Therefore, the terminal 6 is insulated from the weight 8 by interposing the insulating sheet 7. Thus, the detection signal from the piezoelectric element 5 is obtained from the output terminal 11 via the terminal 6. Since the insulating tube 12 is arranged around the cylindrical portion 1c of the bush 1, there is no danger of short-circuiting between the terminal 6 and the cylindrical portion 1c of the bush even when there is some deflection on the terminal 6 when it is assembled. Therefore, provision of the insulating tube 12 is important to obtain a correct detection signal.
Since the vibration detecting device as shown in FIG. 5 has strong mechanical connection between the casing 2 and the weight 8, undesired vibrations are transmitted from the casing 2 of a resinous material to the weight 8 whereby a distortion takes place in the inertial stress. Accordingly, a distortion takes place in the output voltage of the vibration detecting device even though vibrations having a constant amplitude are applied to the seating surface 1a. When the output voltage has such distortion, a high output is produced at a high frequency side, and a flat frequency characteristic can not be obtained even in a low frequency region as shown in FIG. 6a and 6b.
In order to eliminate the above-mentioned problem, there has been proposed to use the cushioning material 21 between the casing 2 and the weight 8 as shown in FIG. 7. The cushioning material 21 cuts undesired vibrations from the casing 2 to the weight 8 and prevents the weight 8 from transmitting an inertial stress which causes uneven output characteristic. An inertial stress which collectly follows the vibrations applied to the device is given to the piezoelectric element 5, whereby an output voltage having a flatter frequency characteristic as shown in FIG. 8 is obtainable. As the cushioning material 21, a resinous material having a DURO METER hardness of at most A-50 is preferably used. However, a resinous material having a DURO METER hardness more than A-50 may be utilized depending on a requsitie frequency characteristic level. A sufficient effect can be obtained by using rubber or an epoxy resin which is softer than polybutylene terephtalate used for the casing 2. For instance, by suitably changing the shape of the output terminal 11 and the metallic terminal 6 and by suitably arranging the cushioning material 21 around the entire circumferential portion of the weight 8, a further excellent effect can be obtained (in comparison with the embodiment as shown in FIG. 7b wherein the cushioning material 21 is arranged at the portion except the connecting portion of the output terminal 11 and the terminal 6). In order to obtain further effect, the cushioning material 21 is extended to the area of the nut 9 to isolate the vibrations from the casing 2 so that the undesired vibrations are not transmitted to the weight 8.
In the conventional vibration detecting device as shown in FIG. 7, the frequency characteristic can be improved to some extent. However, it is insufficient to isolate undesired vibrations by arranging the cushioning material around the weight because the mechanical connection between the weight and the casing is still strong. In the conventional vibration detecting device, uneven frequency characteristic curve as shown in FIG. 8 is found and an output voltage varies depending on frequencies.